LAUSR

Abstract Asian soybean rust (ASR) is the principal disease that affects the soybean crop. Fungicides are most effective in controlling ASR. However, the intrinsic factors related to the soybean cultivar, suitability of the application technology to the target (leaves) and environmental factors directly affect the application effectiveness control. This study investigated the effect of the combined use of spray volumes (70, 110 and 150 L ha−1) and droplet sizes (fine and medium) on the chemical control of ASR in two cultivars with different leaf area indices (LAI) [BMX Lanca cv. (LAI 6.0)]. For the BMX Lanca, in 2017, when the maximum LAI achieved was 5.8, the use of increasing spray volumes, according to the crop growth, and fine droplets (in the applications 2 and 3) presented an area under the disease progress curve (AUDPC) and productivity results similar to the use of the 150 L ha−1 rate for all the applications, irrespective of the droplet size employed. For the BMX Garra, which reached a maximum LAI of 7.1 in the same year, the highest seed yield and lowest AUDPC were obtained for the spray volume of 150 L ha−1 in all the applications, using both droplet sizes. In 2018, when the LAI of the cultivars were reduced because of lack of rainfall, BMX Lanca and BMX Garra showed maximum LAI of 4.9 and 5.5, respectively, and the same treatments that stood out in the 2017 season for both cultivars presented the lowest values regarding the AUDPC. Otherwise, none or minor statistical differences were detected between the combinations of spray volumes and droplet spectra for both cultivars in 2018. The spray volume and droplet size should be determined according to the leaf area of the crop at the time of the application and under conditions of disease pressure.